US President Donald Trump defended his firing of FBI Director James Comey and said he did not ask Comey to end his probe into former National Security Adviser Michael Flynn.

OPINION: What's been missing so far in the scandals surrounding the Trump White House is a concrete act taken at the behest of foreign powers.

Now there's strong evidence of one: former Trump US national security adviser Michael Flynn reportedly stopped an attack on the Islamic State capital of Raqqa by Syrian Kurds, a military action strongly opposed by Turkey, after receiving more than US$500,000 in payments from a Turkish source.

The Kurds' offensive had been greenlit by former US President Barack Obama's administration, and was now back on track, approved again by President Donald Trump sometime after Flynn was fired.

CARLOS BARRIA/REUTERS

Former Trump US national security adviser Michael Flynn reportedly stopped an attack on the Islamic State capital of Raqqa by Syrian Kurds, a military action strongly opposed by Turkey, after receiving more than US$500,000 in payments from a Turkish source.

If this story proves accurate then it's a game changer for special counsel Robert Mueller's investigation.

It demonstrates that, at least while Flynn was national security adviser-designate and until he was fired after 24 days in office, US government policy on a core matter of national security was open to the highest foreign bidder.

REUTERS

Citing a memo, a source says Donald Trump asked James Comey to end a federal probe on former national security adviser Michael Flynn’s ties with Russia.

That's a form of bribery that could land Flynn in prison and, potentially, give Mueller leverage to get Flynn to testify about whatever else he knows.

To understand the significance of the Flynn story, you have to start with the convoluted politics of the Middle East. The US has been trying to defeat Islamic State for several years now, and has been repeatedly stymied by the lack of ground forces willing to attack the militant group, especially in Syria.

The exception is the Syrian Kurds. As I wrote just three days ago in a column that (I thought!) had nothing to do with Flynn, the Syrian Kurds are now seeking to take Islamic State territory - particularly the capital of Raqqa - which they hope to exchange with Syrian President Bashar al-Assad for regional autonomy.

KEVIN LAMARQUE/REUTERS

US President Donald Trump apparently tried to protect Michael Flynn from an FBI probe.

The US wants the Syrian Kurds go for it; so does Russia. The only serious objector is Turkey, which fears the creation of an autonomous Kurdish region in Syria and hates the Syrian Kurdish militia, which the Turks associate with anti-Turkish Kurdish forces in their country.

Got it? Good, because you'll need this background to see just what Flynn did and why it was so bad.

According to the story reported Wednesday by the McClatchy news service, 10 days before Trump was sworn in, Obama National Security Adviser Susan Rice informed Flynn, Trump's national security adviser-designate, that the Obama administration had endorsed the Syrian Kurdish offensive on Raqqa.

But because the Obama administration knew that the attack almost certainly wouldn't happen until after Trump became president, Rice asked Flynn for the go-ahead.

Flynn immediately said no. As a direct result, the attack was put on hold.

Trump didn't reverse that decision until some weeks after Flynn was fired.

Now, more than four months after the Rice-Flynn conversation, the offensive still hasn't taken place.

What makes Flynn's refusal so outrageous - and likely criminal - is that, as we now know, Flynn had in August gotten a more than US$500,000 contract from Turkish businessman Ekim Alptekin, the chairman of the Turkey-US Business Council who has close ties to President Recep Tayyip Erdogan.

Nominally, the contract was for public relations and research work vilifying exiled Turkish cleric Fethullah Gulen, Erdogan's personal bogeyman.

However, it's straightforward to infer that a contract of that size given to a man who was so central to the Trump campaign was intended to buy favour in case Trump was elected.

By blocking the Raqqa attack, Flynn directly served Turkish national interests the moment he had a chance to do so.

Although he wasn't technically in office when he spoke to Rice, Flynn apparently continued to pursue the pro-Turkish policy once the Trump administration was in place.

That's a crime, or rather several crimes.

The Hobbs Act criminalises quid pro quo bribery of a public official.

Flynn may or may not have been a public official during the transition, depending partly on whether he was being paid by government funds, but he was acting as a public official when he spoke to Rice, and he was fully one once he became national security adviser.

It's also possibly wire fraud, the generic federal crime of a scheme to get money under false pretenses.

Of course, Flynn could defend himself against a criminal charge by saying that he simply thought that the Trump administration should reconsider Obama's Syria policy in its entirety.

The trouble with this defence is that Trump authorised the Raqqa attack after Flynn was gone.

Given that the Obama administration made the same judgment before Flynn came into the picture, it looks very much like the only barrier to the policy was Flynn, who had been paid by Erdogan's Turkey.

To make matters worse, Flynn has otherwise been obsessed with fighting radical Islamic terrorism. His decision supporting the Turkish position specifically blocked an attack on the capital of what is now the worst Islamic terrorist entity in the world.

The case against Flynn is circumstantial, but the circumstances are damning.

What's most significant here is that there's a clearly explainable, substantive crime that could be charged against Trump's most important national security appointee - and it involves actually doing something on behalf of a foreign power.

That's different from simply failing to disclose payments, or even trying to make an investigation go away, the way Trump apparently tried to protect Flynn from an FBI probe.

Although a cover-up can bring down a president, it's not very satisfying for an investigation to find lots of smoke but no fire.

If the story turns out to be true, Flynn's decision looks very much like he sold out US national security to Turkey, weakening the US in the war on terrorism. That's a fire - and Mueller's investigation will have to get to the bottom of it.

- Noah Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to US Supreme Court Justice David Souter. His seven books include "The Three Lives of James Madison: Genius, Partisan, President" and "Cool War: The Future of Global Competition."